1. Field of the Invention
Embodiments of the present invention relate to novel systems and methods for processing semiconductor substrates. More specifically, aspects of the present invention relate to systems and methods for enhancing plasma processing of a semiconductor substrate.
2. Description of the Related Art
Plasma-generating reactors have been used extensively in processes for fabricating integrated circuit and microelectromechanical (MEM) devices on or from a substrate such as a silicon wafer. One particularly useful reactor is the inductively-coupled plasma-generating (ICP) reactor, which inductively (and to some extent capacitively) couples radio frequency (RF) power into a gas contained within the reactor to generate a plasma. The plasma contains species such as ions, free radicals, and excited atoms and molecules that may be used to process the substrate and ultimately produce integrated circuit or MEM devices.
An ICP reactor may be used to carry out a variety of processes to fabricate integrated circuit devices on a semiconductor substrate, including anisotropic and isotropic etching and chemical vapor deposition (CVD). For anisotropic etching, an ICP reactor may be used to produce a plasma with a high ion density. Generally, a low pressure and high RF power are used which favor the production of ions. The ions are accelerated perpendicularly toward the surface of the substrate by an electric field which is typically induced by an RF bias applied to the substrate support. The ions bombard the substrate and physically and/or chemically etch the substrate and any materials deposited thereon, such as polysilicon (poly), silica (SiO2, silicon oxide, or oxide), silicon nitride (Si3N4 or nitride), photoresist (resist), or metals. Such anisotropic etching processes are useful for forming integrated circuit features having substantially vertical sidewalls.
ICP reactors are also useful for producing reactive species for isotropic etching, particularly for stripping photoresist from the surface of a semiconductor substrate. Sufficient energy is coupled into the gas in the plasma generation chamber to form a plasma containing a high density of atomic and molecular free radicals that chemically react with the polymeric photoresist to facilitate its removal. For example, a plasma may be used to dissociate oxygen gas into atomic oxygen that reacts with polymeric photoresist to form CO and CO2, which evolve and are carried away by the process gas into the exhaust system of the reactor. In such processes, in contrast to anisotropic etching, it is often desirable to reduce or eliminate ion bombardment which may damage the surface of the substrate.
ICP reactors are also useful for CVD of a material onto the surface of a substrate. For many CVD processes, the process is enhanced by ion bombardment and may be carried out at lower temperatures with higher deposition rates by exposing the substrate directly to the plasma (this process is called plasma-enhanced or plasma-assisted CVD). In plasma-enhanced chemical vapor deposition (PECVD), sufficient energy is coupled to the gas in the plasma generation chamber to form a plasma containing a high density of atomic and molecular free radicals and energetic species that interact with the surface of the substrate to form a deposited layer. For example, silane (SiH4) releases hydrogen and can be used to deposit a layer of polysilicon onto a substrate. In addition, silane or tetraethoxysilane (TEOS) can be added to an oxygen plasma to deposit a layer of silicon dioxide on a substrate, which in turn can be used as an etch mask during reactive-ion etching or as an insulating layer in circuit devices.
In each of the above processes, processing uniformity can be a critical factor in determining integrated circuit quality, yield, and production rate. Uniform etching, stripping, or chemical deposition over the surface of a wafer assures that structures fabricated at the center of the substrate""s surface have essentially the same dimensions as structures fabricated near the edge of the substrate. Thus, the yield of chips from a wafer depends, at least in part, on the etch, strip, or deposition uniformity across the wafer""s surface. Higher yield also contributes to a higher production rate.
Processing uniformity may be affected by the density and distribution of reactive species in the plasma and by the plasma potential across the substrate""s surface. Processing may occur at higher rates in areas of the wafer surface where there is a higher density of reactive species. Further, for ion enhanced processes, any variance in the plasma potential across the wafer""s surface will cause a corresponding variance in ion bombardment energies which may, for example, lead to nonuniform ion etch or ion enhanced deposition.
A number of different inductively-coupled reactor configurations have been used to produce plasmas for the processing of a variety of substrate sizes. In an effort to increase chip production rates, however, integrated circuit manufacturers have moved from small-diameter substrates to substrates of ever-increasing diameters. At one time, 100 millimeter (mm) silicon wafers were the norm. These wafers were subsequently replaced by 150 mm and then 200 mm wafers; most sizes are currently being replaced by 300 mm wafers that will undoubtedly become conventional for high volume and high complexity computer chips in the near future. In time, it is expected that even larger wafers will be developed.
With larger diameter substrates, it becomes difficult to produce a plasma with highly uniform properties in a conventional reactor chamber. For ion enhanced processes, the flux of ions at locations across the wafer surface may become nonuniform. FIG. 1 illustrates a typical cylindrical ICP reactor, generally indicated at 100. In reactor 100, gas is provided to the reactor chamber 102 through an inlet 104. A helical induction coil 106 surrounds the chamber and inductively couples power into the gas in reactor chamber 102 to produce a plasma. Ions or neutral activated species then flow to a wafer surface 108 for processing. In addition, an RF bias may be applied to the wafer to accelerate ions toward the wafer surface for ion enhanced processing.
The dashed line 110 in FIG. 1 represents a maximum potential surface (MPS) for a plasma produced in reactor 100. An MPS is a geometric construction of the maximum values of the DC plasma potential along arbitrary lines drawn from the substrate to points on the interior surfaces of chamber 102. An ion which is created above the MPS senses an electrostatic potential that tends to drive it toward the interior walls of the chamber. An ion created within the MPS senses an electrostatic potential that tends to push it toward the substrate. A higher percentage of ions near the edges of the wafer are driven to the walls than near the center of the wafer as illustrated by the dome-like MPS 110. The difference in the ion flux between the edges and the center of the wafer may be significant and lead to nonuniform processing.
The shape of the MPS may be influenced by the configuration of reaction chamber 102. FIG. 2 illustrates a schematic diagram showing the plasma properties in a reactor that contains a conically-shaped section 202 above a vertical-walled section 204 of a reactor generally indicated at 200. The dashed line 210 in FIG. 2 represents the MPS for a plasma produced in reactor 200. Also shown in FIG. 2 is an induction coil 220 positioned along conically-shaped section 202 of the reactor. This configuration produces regions of xe2x80x9chot electronsxe2x80x9d generally indicated at 225 in the chamber, with the hot electrons producing a particularly high rate of ionization of the processing gas in these regions of the chamber. The high rate of ionization helps to counteract the natural tendency of the MPS to drop off near the sidewalls of the reactor. The result is the development of a flatter MPS in the chamber than would have been attained in a reactor having no conical section, and a more uniform ion density above the substrate is achieved as well. In addition, the truncated conical arrangement of the coil allows the top of the chamber 230 to be lowered (moved toward the substrate) which helps flatten out any peak in the stagnation surface over the center of the wafer.
A conical reactor, such as reactor 200, provides advantages over a reactor with vertical sidewalls such as reactor 100. The MPS in a conical reactor has less curvature (i.e., less of a dome-shape) than would have been the case in a reactor without conical walls. However, it is desirable to flatten the MPS even further, and to provide plasma processing properties that are enhanced to an even greater extent.
What is needed is a plasma reactor with enhanced control over the plasma characteristics while allowing large diameter wafers to be processed. Preferably such a plasma reactor can be used to provide a uniform plasma potential and/or species concentration across the surface of a substrate for etching, stripping or chemical vapor deposition and can be used to process smaller substrates such as 100 mm, 150 mm, and 200 mm wafers as well as 300 mm or larger wafers. In addition, for non-ion enhanced processes, such as photoresist strip, it is desirable to provide a reactor configuration that both enhances the uniform production of reactive species and provides a plasma generation volume that can be used to isolate the plasma from the wafer surface to reduce ion drive-in.
Aspects of the present invention achieve enhanced plasma uniformities by providing a plasma shaping member within the reaction chamber of an ICP reactor. In an exemplary embodiment, the plasma shaping member extends from the bottom surface of a portion of the top wall of reaction chamber. Preferably, the plasma shaping member is generally centered above the substrate support. The plasma shaping member may be a separate piece of hardware attached to a portion of the bottom surface of the top wall of the chamber, or it may comprise an integral part of the top wall itself. The plasma shaping member may also be configured to have a recessed or concave central region and an extended portion peripheral to the central region. Both the extended and recessed segments of this exemplary embodiment face the substrate. The term xe2x80x9cextendedxe2x80x9d as used in this exemplary embodiment is meant to indicate that a portion of the plasma shaping member protrudes from a region of the top wall of the chamber into the volume of the chamber body to a greater degree than does the recessed region.
In another exemplary embodiment, the exemplary plasma shaping member has dimensions X, Y, Z, and R that are relevant to the design and performance of the member. The exemplary plasma shaping member may be thought of as a two-part structure comprising a toroidal-like form adjacent to a substantially cylindrical structure having a similar outside diameter as the toroid-like form. The recessed portion of the exemplary plasma shaping member is that portion of the substantially cylindrical structure that would lie within the toroid in a projected (plan) view. The recessed region of the plasma shaping member has thickness X (which may be approximately the same dimension as the height of the cylindrical structure of the member). The extended portion of the exemplary plasma shaping member (the toroidal-like form) has a width Z and height Y. The dimension R indicates the outside radius of the member. However, the extended portion is not required to be the outermost annulus of the member, nor does it have to be rectangular or square in cross section.
In another exemplary embodiment, the width of the extended portion of the plasma shaping member (Z) has a value greater than or equal to about 10 percent of the outside diameter of the member (2R), and a value less than or equal to about 30 percent of the outside diameter of the member. The minimum value of X plus Y, the sum of which is the distance the member protrudes into the chamber, may be about 10 percent of the height of the chamber (Hchamber). In one embodiment of a reactor with a plasma shaping member, the distance from the top of the substrate support to the bottom of the plasma shaping member, which algebraically would be Hxe2x88x92(X+Y), is four inches or less. The outside diameter of the plasma shaping member may range from about 60 to about 90 percent of the diameter of the substrate (Dsubstrate).
The plasma shaping member may be fabricated from essentially any type of material, including quartz, metals, ceramics, and coated metals, and combinations thereof. The material may be selected taking into account considerations such as cost, machinability, and potential for contamination. The material should also be selected, and the plasma shaping member should be configured, to provide an electrically floating potential relative to ground while processing the substrate. Providing an electrically floating potential during processing is one of the mechanisms by which the member removes ions from the chamber and serves to shape the plasma. Positive ions that hit the surface of the shaping member recombine with an electron (or possibly negative ion) to form a neutrally charged species. The recessed section and the extended section of the exemplary plasma shaping member both function to provide a surface upon which positive ions may become converted to electrically neutralized particles.
The exemplary plasma shaping member may further reduce the ion density of a plasma by a second mechanism. The extended section physically blocks the diffusion of high temperature electrons from regions adjacent to coil (where the high temperature electrons are generated) to the center regions of the chamber that overlie the center of the substrate. If these electrons are free to diffuse to the center of the chamber, they may cause additional ionization of the process gas beneath the recessed region of the member overlying the center of the substrate. Because the extended section of the exemplary plasma shaping member provides an obstacle to the diffusion of high temperature electrons from the edge of the chamber to the center of the chamber, it will be appreciated by one skilled in the art that virtually any shape of protrusion will suffice.
As a result, a reactor according to exemplary embodiments of the present invention produces a plasma with a more uniform potential and ion concentration across both the center and periphery of the substrate surface. For example, it is believed that exemplary embodiments utilizing the principles of the present invention can achieve a plasma uniformity that is better (less than) xc2x115 percent. As a result, an RF bias applied to wafer support accelerates ions toward the wafer surface for etching or plasma-enhanced CVD with a substantially uniform energy and density distribution. This provides a consistent processing rate (whether for etch or deposition) across the surface of the substrate.